1. Field of the Invention
The present invention relates to semiconductor packaging. More particularly, the invention relates to an apparatus and method of packaging a semiconductor to dissipate heat generated within the semiconductor package.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Integrated circuits, such as microprocessors and memory devices, are used in a wide variety of applications. Such applications include personal computers, industrial control systems, telephone networks, and a host of consumer products, just to name a few. As most people are aware, an integrated circuit is a highly miniaturized electronic circuit that has revolutionized the functionality, dependability, and size of these various products.
In the manufacturing of integrated circuits, numerous microelectronic circuits are simultaneously fabricated on a semiconductor substrate. Such substrates are typically referred to as wafers, and a typical wafer includes a number of different regions, known as die regions. When the fabrication of the integrated circuits on the wafer is complete, the wafer is cut along these die regions to form individual die. Each die contains at least one microelectronic circuit, which is usually replicated on each die.
To use such a circuit in an electronic product, the die is typically placed within a sealed package. The sealed package performs several basic functions, such as electrically coupling the die to external electrical circuits, protecting the die from physical or environmental damage, and dissipating the heat generated within the die. One of the main functions of the package is to allow electrical connection of the die within the package to a circuit board or other external electrical components. Typically, these external electrical connections cannot be made directly to the die because of the thin and fragile bond wires used to interconnect the components on the die's surface. The diameters of the thinnest wires available are many times larger than the bond wires used on the surface of the die. Therefore, the die wiring typically terminates in larger bonding pads. The larger bonding pads allow external wires to be electrically connected to the die.
One method of integrated circuit packaging is known as lead-on-chip (LOC) packaging. LOC packages are generally used for very-large-scale integrated (VLSI) circuits or larger. Typically, the initial component in the packaging process is a leadframe. The leadframe provides the leads for the final package. The bonding pads formed on the die are electrically connected to lead fingers on the leadframe using fine bond wires. In an LOC package, the bonding pads are arranged down the center of the die. Typically, a double-sided adhesive tape is used to attach the die to the lead fingers of the leadframe. One side of the tape is applied to the underside of the leadframe. The die is then attached to the adhesive tape using pressure. Additionally, the lead fingers support the die during the encapsulation process.
A typical encapsulation material used in integrated circuit packaging is molded-epoxy. In a typical encapsulation process, a molding machine is charged with warmed pellets of the epoxy material. The molding machine forces a ram into the pellets to produce a liquid state in the epoxy. The liquid epoxy is then forced around the die on the leadframe. After the epoxy sets in the molding machine, the integrated circuits in their molded-epoxy packages are removed and placed in an oven for final curing.
Another function of the sealed package is to protect the die from physical or environmental damage. Physical damage can result in a myriad of ways, including jarring, physical abuse, and/or particle contamination. Environmental damage can result from chemicals, moisture, and even gases. Protection of the die is accomplished by securing the chip to a substrate or die-attachment area, and the die and substrate are then surrounded by an encapsulation material. The encapsulation material seals and protects the die and substrate within a protective cocoon.
Thermal management is another important factor in package design because some die can generate substantial amounts of heat during operation that, if not removed, will damage the die. The prime mechanisms of heat transfer involved with the thermal management of integrated circuit packages are conductive heat transfer and convective heat transfer. Conductive heat transfer, or conduction, is the process by which heat diffuses through a solid or a stationary fluid. In conduction, energy is transported from a region of higher temperature to a region of lower temperature by the drift of electrons. The property used to measure a materials ability to conduct heat is known as “thermal conductivity.” A larger value of thermal conductivity indicates that a material is a better conductor of heat. For example, air is not a particularly good conductor of heat and has a corresponding thermal conductivity of 0.024 W/mK. Epoxies are slightly better conductors at 0.23 W/mK. Thermal greases are better conductors at 1.10 W/mK. Metals, however, are very good conductors of heat. Gold, for example, has a thermal conductivity of 298 W/mK. Two other metals have even higher thermal conductivities: copper at 395 W/mK and silver at 419 W/mK.
Convective heat transfer, or convection, is the process by which a moving fluid transfers heat to or from a surface. The motion of the fluid augments of the transfer of heat in convection. If the flow of fluid is not forced by a fan or pump, the mechanism of heat transfer could be either through conduction or natural convection. Natural convection can occur when the fluid near the surface heats up. The warmer fluid will be less dense than the cooler fluid above it and will rise. Thus carrying away heat. The process will continue as warmer fluid is displaced by cooler fluid.
Instead of the thermal conductivity (k) of the material, a term called the heat transfer coefficient (h) is used to simplify convection heat transfer calculations. The determination of the heat transfer coefficient is a very complicated matter because the coefficient can be affected by many factors. One factor is whether the convection is natural convection or forced convection. Forced convection is produced by a fan or pump forcing the flow of fluid. Generally, forced convection is a much better mechanism of heat transfer than natural convection. Another factor is the type of fluid flow, i.e., whether it is smooth laminar flow or whether it is violent turbulent flow.
The geometry of the body is also very important, i.e. whether the fluid is flowing over a flat plate, a sphere, or inside a pipe. The fluid involved in convection is a very important factor. All other factors being the same, the heat transfer coefficient will usually be much higher if water, rather than air, is the fluid transferring the heat.
It is possible for an integrated circuit to experience a catastrophic thermal failure. Catastrophic thermal failure is an immediate, thermally induced, total loss of electronic function. In most integrated circuits, heat is transferred from the die through the package by conduction. Heat is then removed from the surface of the package, typically, by convection to a ventilated enclosure. Therefore, the thermal properties of the material forming the package are an important factor in removing heat from the die. In some cases, the integrated circuits generate large quantities of heat that require greater amounts of heat to be transferred than can be achieved by conduction through the package without damaging the die.
Various methods have been used to assist the removal of heat generated within an integrated circuit. These methods may range from the use of a simple heat sink, to the use of fans, to even the use of cyrogenics, such as liquid nitrogen. Substantial differences in the performance and cost of the heat transfer methods, as well as the integrated circuits, guides the selection of a particular approach. However, the use of heat sinks is a commonly used method to remove heat from many integrated circuit packages. Generally, heat sinks are made of metal and shaped to include fins. Metals are used because metals, as described above, are generally good conductors of heat. The fins serve to increase the surface area in contact with the coolant, thereby increasing the heat transfer to the coolant. The heat sink is attached to the outer surface of the package. The heat produced within the die is transferred to the package by conduction. The heat is then transferred by conduction from the package to the heat sink. Finally, the heat is removed from the heat sink by either convection or conduction to the air surrounding the heat sink.
Before a heat sink can transfer heat to a fluid, the heat produced within the die must first be transferred through the package to the heat sink. A temperature gradient will be produced in the package with a higher temperature on the inside of the package near the die and a lower temperature on the outer surface of the package. For the same amount of heat transfer, a lower thermal conductivity (k) will produce a higher temperature gradient and, consequently, a higher die temperature. Unfortunately, integrated circuit package material usually consists of molded-epoxy or ceramic materials that are not good conductors of heat and have low thermal conductivities. Thus, current packaging materials produce a higher die temperature than could be produced if higher thermally conductive material were used.
There is a need, therefore, for an improved technique for removing the heat generated by a semiconductor device. In particular, there is a need for an improved system for removing the heat generated by a semiconductor device within a sealed package.